The present invention relates to a differential limiting force control system for controlling a differential limiting force for limiting a differential action between right and left drive wheels of a vehicle by controlling an engagement force of a differential limiting clutch of a limited slip differential. More specifically, the present invention relates to a differential limiting force control system responsive to a turning radius and a centripetal acceleration of a vehicle.
Japanese patent provisional publication No. 61-67629 discloses one conventional example. A control system of this example is arranged to increase an engagement force of a differential limiting clutch as the vehicle speed increases. Japanese patent provisional publication No. 61-102321 discloses another conventional control system, which is arranged to sense an accelerating condition of a vehicle, and to increase a clutch engagement force of a differential limiting clutch in rapid accleration.
However, these conventional control systems have a disadvantage in that the clutch engagement force is controlled in accordance with only the vehicle speed, or the condition of acceleration. If control characteristics are adapted to a high friction coefficient road condition so that the clutch engagement is made relatively strong, then these conventional systems tend to cause rear end swing of the vehicle by a steep increase of the differential limting force during driving on a low friction coefficient road surface, and produce strong understeer at the time of "turn-in" during turn at a tight corner on a road surface of a low friction coefficient. If the control characteristics are adapted to a low friction coefficient road condition by making the clutch engagement force relatively weak, then the differential limiting force becomes so low during driving on a high friction coefficient road surface, that spin of an inside wheel of the turn is increased significantly, and the ability of acceleration during a turn is degraded.
It is possible to adjust the control characteristics in accordance with the friction coefficient of a road surface. However, it is difficult to accurately sense the friction coefficient.
Japanese patent application No. 61-217654, first publication number 63-71428, proposes a control system using a centripetal acceleration to adjust control based on the vehicle speed and accelerating condition in such a manner that the differential limiting force is increased as the centripetal acceleration increases. However, this control system is still disadvantageous in the following points. In a high speed straight ahead operation, the vehicle speed is high but the centripetal acceleration is low. In such a situation, the differential limiting force is insufficient, so that the stability against side wind is poor. This system decreases the differential limiting force almost to zero in a low speed straight ahead operation. Therefore, this system tends to produce spin during driving on a split mu (friction coefficient) road having different friction coefficients between a contact surface of the left wheels and a contact surface of the right wheels.